The present invention generally relates to an optical recording/reproducing method for recording/reproducing an image, audio information, or the like in accordance with optical information, and particularly relates to an optical recording medium and a recording/reproducing method making it possible to perform high density recording/reproducing.
Conventionally, optical disks such as an audio compact disk (CD), a video disk (VD), and the like, on which recording/-reproducing of optical information can be performed have been very widely used.
Although there are certain problems with conventional disk, however. For example, a required disk-drive control system is complicated and expensive, optical disks are widely because they are superior to conventional recording media in many regards. For example, an optical disk recording can be made with much greater density, one or two orders of magnitude, than a magnetic recording because the signal is recorded/reproduced using a small light spot having a size of about the wavelength of light. Also, an optical disk has a signal surface which is not worn during use so that it is superior in non-volatility of recording (holding of recorded data) because the reproduction of the recorded information is performed by a noncontact method using a light beam.
In conventional optical disks, the signal is recorded as a series of binary values indicated by whether a hole (pit) exists or not in a light reflection surface of the disk, and information is expressed by a pit length (size) and a pit interval. There are two recording types, one being an analog recording such as laser vision or the like in which the pit length and pit interval vary continuously, the other being digital recording such as a digital compact disk or the like in which the pit length and pit interval discretely vary. In analog recording, generally, although any signal can be recorded so as to obtain an extremely high information packing density, there is a problem concerning the accuracy of the recorded signal and the accuracy of signal reproduction whereby noise in optical and electric systems directly affects the signal-to-noise ratio of the reproduced signal.
In digital recording, on the other hand, the recorded information can be accurately reproduced if the mark length can be discriminated. Further, digital recording has the advantage that even if a certain amount of error exists, the original correct information can be restored using an error correcting code. In digital recording, however, since the mark size must be discretely distributed within a predetermined range of length as described above, the number of marks is unavoidably limited, and it is therefore a matter of course that the recording density is limited.
As described above, in the case of a binary signal formed by a series of pits, a recording method in which the pit size, pit interval, and track interval are made small to thereby improve the recording density has been used conventionally. In making high the recording density, and correspondingly the processing speed, however, there has been a limit to the achievable recording density, even if the size of a recording signal is made as small as absolutely possible, due to the binary nature of the signal and in that a large number of bits (a large number of combinations of pits) is required in order to express complicated information.
In view of this situation, a method using a multi-valued recording signal (multi-recording) has been proposed in order to further improve the recording density. With respect to this method, a photochemical hole burning method (PHB) is considered to be promising. This method is, however, very difficult to be put to practical use at present because it is necessary to very precisely control the temperature of the recording apparatus.
Also, for example as disclosed in Japanese Patent Unexamined Publication No. Hei-1-319134, there has been proposed a method in which the mark diameter is changed by changing the quantity of light (the quantity of energy) of the recording laser light to thereby record a multi-valued signal on the basis of the light reflection factor corresponding to the value of the mark diameter.
In this method, however, the changes in the recording signal are difficult to distinguish from the case where the signal changes in a continuous manner. The method is therefore not significantly different from the previously described conventional techniques. More specifically, multi-valuing is carried out using a plurality of threshold values of the quantity of recording light as determined by changes in the recording spot diameter. However, the output power level of the recording signal beam generating laser generally unavoidably fluctuates in a range of about 1 to 2 mW, so that an error is caused in the recorded mark diameter. Since as the number of available multi-level values is increased the intervals of the threshold values become less, at a certain number of multi-value levels the fluctuations in the recording beam power level make it impossible to accurately distinguish between adjacent recorded levels.